1. Field of the Invention
The present invention relates generally to switches. More specifically, the present invention relates to switches of the sort in which two conductive surfaces touch.
2. Description of the Related Art
Switches of the sort in which two conductive surfaces touch are used in such technologies as resistive touchscreens and membrane switches. For reasons of convenience and clarity, this background section describes resistive touchscreens.
Touchscreens are used in the computer arts to input data. Rather than a user of a computer inputting data by using a keyboard or a mouse, the user inputs data via a touchscreen mounted on the monitor of the computer. Several patents have been issued regarding resistive touchscreens, including U.S. Pat. Nos. 4,306,110; 5,438,168; 5,541,370; 5,589,857; 5,818,430; and 5,844,175. The basic configuration of a resistive touchscreen comprises a coversheet generally comprising a first substrate, and a second substrate to which the coversheet is bonded. The first substrate, i.e. the substrate of the coversheet, comprises a thin layer of a plastic, such as polyethylene terephthalate (PET). The first substrate has opposed first and second surfaces, the first surface facing a user of the computer and the second surface facing the second substrate. The second surface of the first substrate is coated with a conductive element. The second substrate is generally made of glass, although other transparent plastic materials could be used, and is either flat or curved to fit the curvature of a computer monitor. The second substrate is coated with a conductive element applied to a surface of the substrate facing the first substrate, and thus also facing a user of the computer.
The conductive elements typically are transparent conductive coatings of thin layers of metals or metal oxides such as indium tin oxide (ITO), although other metals or metal oxides could be used. The composition and application of ITO coatings are described in U.S. Pat. Nos. 5,776,373; 5,851,642; and 6,042,752.
The touchscreen also comprises a plurality of spacer dots applied to the conductive coating on one of the facing surfaces, either of the coversheet or of the substrate. Spacer dots are described in U.S. Pat. No. 5,220,136, although other spacer dots could also be used. These spacer dots separate the coversheet from the second substrate. Touching the coversheet presses the two conductive elements, one on the coversheet and one on the second substrate, together and thereby completes an electrical circuit that also comprises conductive elements such as wires on the edges of the conductive coatings. The specific location that is touched, and thus the location of the conductive elements that come into contact, is the data created and transmitted by the touch.
A problem with traditional touchscreens is the interrelationship between the mechanical properties of the substrate of the coversheet and the coating of the coversheet. The traditional materials used for the coating are generally brittle and can crack and disbond from the flexible plastic coversheet substrate when the substrate is deformed repeatedly. This in turn destroys the continuity of the electric current carrying pathways in the conductive coating, which in turn results in failure of the touchscreen to accurately report the location of a touch. What is needed is a transparent conductive coating that is flexible and durable and will bond to the coversheet substrate for the life of the touchscreen.
In addition to the aforementioned shortcomings of metal and metal oxide coatings, ITO coatings have a limited lifespan, i.e. they fail to transmit the requisite electrical current after a number of touches. ITO coatings are also relatively expensive and have low resistance to mechanical damage. What is needed is a transparent conductive coating that has a longer lifespan, a low cost, and is resistant to mechanical damage.
The present invention solves the above problems by changing both the material used in the transparent conductive coating on the coversheet and the electrical characteristics of the electrical circuit.
In a first improvement over traditional touchscreen technology, the present invention uses a layer of an intrinsically conductive polymer as the coating on the coversheet. Intrinsically conductive polymer coatings on flexible plastic coversheets are less brittle, more flexible, more resistant to mechanical damage, and available at lower cost than ITO coatings on plastic coversheets; further, by coating the plastic coversheet with an intrinsically conductive polymer coating rather than with a metallic coating, the coating does not crack or disbond from the substrate, and thus has a longer life.
Intrinsically conductive polymers known in the art include polyacetylene, polypyrrole, polyaniline, polythiophene, etc. More details about suitable intrinsically conductive polymers can be found in textbooks, such as xe2x80x9cAdvances in Synthetic Metalsxe2x80x9d, ed. P. Bernier, S. Lefrant, and G. Bidan, Elsevier, 1999; xe2x80x9cIntrinsically Conducting Polymers: An Emerging Technologyxe2x80x9d, Kluwer (1993); xe2x80x9cConducting Polymer Fundamentals and Applications, A Practical Approachxe2x80x9d, P. Chandrasekhar, Kluwer, 1999; and xe2x80x9cHandbook of Organic Conducting Molecules and Polymersxe2x80x9d, Ed. Walwa, Vol. 1-4, M. Dekker Inc. (1997).
In order to enjoy the benefits of the improved mechanical properties of intrinsically conductive polymer coatings over metal or metal oxide coatings, it has been discovered that it is necessary to carefully control the excitation of the electric circuit used with touchscreens incorporating intrinsically conductive polymers. This careful control of the excitation of the electric circuit is not required with conventional metal or metal oxide coatings such as ITO, and does not provide any additional advantage for these conventional materials.
In particular, both the level of the excitation voltage and the polarity of the voltage applied to the intrinsically conductive polymer coating must be carefully controlled to maximize the benefit of the improved mechanical properties of the intrinsically conductive polymer coatings. The excitation voltage of the touchscreen must be reduced from the 5 volts dc that is commonly used with many commercially available touchscreens. The same effect is observed with ac voltage. Additionally, the durability of the touchscreen is significantly improved when the intrinsically conductive polymer coating is held at a negative potential. These surprising effects are not observed with conventional metal or metal oxide coatings.